Add imagecache node association to mm_dir

This patch adds an option to the mm_dir configuration to try to find the nid that is associated with the original file, if the image being harvested was created with imagecache.

Here's the use case:

You have two configurations, one that harvests original images from cck and another that uses mm_dir to pick up the imagecache versions. Both move your files to S3.

If you have "Delete Media Mover items with nodes" selected, your imagecache versions will stay on S3 because they dont know what node they are associated with.

This option should fix that.